1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to transmitting and receiving legacy format data in a high throughput wireless network.
2. Description of the Related Art
Recently, there has been an increasing demand for ultra high-speed communication networks due to widespread public use of the Internet and a rapid increase in the amount of available multimedia data. Since local area networks (LANs) emerged in the late 1980s, the data transmission rate over the Internet has drastically increased from about 1 Mbps to about 100 Mbps. Thus, high-speed Ethernet transmission has gained popularity and wide spread use. Currently, intensive research into a gigabit-speed Ethernet is under way. An increasing interest in the wireless network connection and communication has triggered research into and development of wireless LANs (WLANs), and greatly increased availability of WLANs to consumers. Although use of WLANs may reduce performance due to lower transmission rate and poorer stability as compared to wired LANs, WLANs have various advantages, including wireless networking capability, greater mobility and so on. Accordingly, WLAN markets have been gradually growing.
Due to the need for a higher transmission rate and the development of wireless transmission technology, the initial Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, which specifies a transfer rate of 1 to 2 Mbps, has evolved into advanced standards including IEEE 802.11a, 802.11b and 802.11g. The IEEE 802.11g standard, which utilizes a transmission rate of 6 to 54 Mbps in the 5 GHz-National Information Infrastructure (NII) band, uses orthogonal frequency division multiplexing (OFDM) as its transmission technology. With an increasing public interest in OFDM transmission and use of a 5 GHz-band, much greater attention is been paid to the IEEE 802.11g standard and OFDM transmission technology than to other wireless standards.
Recently, wireless Internet services using WLAN, so-called “Nespot,” have been launched and offered by Korea Telecommunication (KT) Corporation of Korea. Nespot services allow access to the Internet using a WLAN according to IEEE 802.11b standard, commonly called Wi-Fi (wireless fidelity). Communication standards for wireless data communication systems, which have been completed and promulgated or are being researched and discussed, include Wide Code Division Multiple Access (WCDMA), IEEE 802.11x, Bluetooth, IEEE 802.15.3, etc., which are known as 3rd Generation (3G) communication standards. The most widely known, cheapest wireless data communication standard is IEEE 802.11b, a series of IEEE 802.11x. An IEEE 802.11b WLAN standard delivers data transmission at a maximum rate of 11 Mbps and utilizes the 2.4 GHz-Industrial, Scientific, and Medical (ISM) band, which can be used below a predetermined electric field without permission. With the recent widespread use of the IEEE 802.11a WLAN standard, which delivers a maximum data rate of 54 Mbps in the 5 GHz-band by using OFDM, IEEE 802.11g developed as an extension to the IEEE 802.11a standard for data transmission in the 2.4 GHz-band using OFDM and is intensively being researched.
The Ethernet and the WLAN, which are currently being widely used, both utilize a carrier sensing multiple access (CSMA) method. According to the CSMA method, it is determined whether a channel is in use. If the channel is not in use, that is, if the channel is idle, then data is transmitted. If the channel is busy, retransmission of data is attempted after a predetermined period of time has elapsed. A carrier sensing multiple access with collision detection (CSMA/CD) method, which is an improvement of the CSMA method, is used in a wired LAN, whereas a carrier sensing multiple access with collision avoidance (CSMA/CA) method is used in packet-based wireless data communications. In the CSMA/CD method, a station suspends transmitting signals if a collision is detected during transmission. Compared with the CSMA method, which pre-checks whether a channel is occupied before transmitting data, in the CSMA/CD method, the station suspends transmission of signals when a collision is detected during the transmission of signals and transmits a jam signal to another station to inform it of the occurrence of the collision. After the transmission of the jam signal, the station has a random backoff period for delay and restarts transmitting signals. In the CSMA/CD method, the station does not transmit data immediately even after the channel becomes idle and has a random backoff period for a predetermined duration before transmission to avoid collision of signals. If a collision of signals occurs during transmission, the duration of the random backoff period is increased by two times, thereby further lowering a probability of collision.
The CSMA/CA method is classified into physical carrier sensing and virtual carrier sensing. Physical carrier sensing refers to the physical sensing of active signals in the wireless medium. Virtual carrier sensing is performed such that information regarding duration of a medium occupation is set to a media access control (MAC) protocol data unit/physical (PHY) service data unit (MPDU/PSDU) and transmission of data is then started after the estimated duration has elapsed. However, if the MPDU/PSDU cannot be interpreted, the virtual carrier sensing mechanism cannot be adopted.
IEEE 802.11n provides coverage for IEEE 802.11a networks at 5 GHz and IEEE 802.11g networks at 2.4 GHz and enables stations of various data rates to coexist. For operating the stations of various data rates using the CSMA/CA method, the stations must interpret MPDU/PSDU. However, some stations, that is, legacy stations, may not often process data transmitted/received at high rates. In such a case, the legacy stations cannot perform virtual carrier sensing.
FIG. 1 is a data structure of a related art format Physical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU) as defined by the IEEE 802.11a protocol. The PPDU includes a PLCP header and Physical Layer Service Data Unit (PSDU). A data rate field 3 and a data length field 4 are used to determine a length of a data field that follows the PLCP header of the PPDU. The data rate field 3 and the data length field 4 are also used to determine the time of the data being received or transmitted, thereby performing virtual carrier sensing. In addition, in a case where a Message Protocol Data Unit (MPDU) is accurately filtered from the received PPDU, a “Dur/ID” field, which is one field among the header fields of the MPDU, is interpreted and the medium is virtually determined to be busy for an expected use time period of the medium. In a case where a preamble field and a signal field of a PPDU frame being received are only erroneously interpreted, media may attempt data transmission by a backoff at a predetermined Extended Inter-Frame Space (EIFS), which is longer than a Distributed Coordination Function (DCF) Inter-Frame Space (DIFS), so that fairness in media access of all stations available in DCF is not ensured.
In a network where an existing station using a conventional protocol or a legacy station and a High Throughput (HT) station coexist, the legacy station may be upgraded for transmission and reception of HT data. However, a legacy station or a conventional station cannot perform virtual carrier sensing because these stations cannot interpret the “Dur/ID” field present in the data which was transmitted and received by the HT station.
FIG. 2 is a diagram illustrating that a legacy station with a low transmission rate is incapable of performing virtual carrier sensing when a plurality of stations having a variety of transmission capabilities coexist.
A transmitter-side high throughput station (abbreviated as transmitter-side HT STA) 101 is a station complying with the IEEE 802.11n protocols and operating using a channel bonding technique or a multiple input multiple output (MIMO) technique. Channel bonding is a mechanism in which data frames are simultaneously transmitted over two adjacent channels. In other words, according to a channel bonding technique, since two adjacent channels are bonded during data transmission, channel extension exists. The MIMO technique is one type of adaptive array antenna technology that electrically controls directivity using a plurality of antennas. Specifically, in an MIMO system, directivity is enhanced using a plurality of antennas by narrowing a beam width, thereby forming a plurality of transmission paths that are independent from one another. Accordingly, a data transmission speed of a device that adopts the MIMO system increases as many times as there are antennas in the MIMO system. In this regard, when data is transmitted/received using the channel bonding or MIMO technique, capable stations can read the transmitted/received data but incapable stations, i.e., legacy stations, cannot read the transmitted/received data. Physical carrier sensing enables a physical layer to inform an MAC layer whether a channel is busy or idle by detecting whether the physical layer has received a predetermined level of reception power. Thus, the physical carrier sensing is not associated with interpreting of data transmitted and received.
If the transmitter-side HT STA 101 transmits HT data, a receiver-side HT STA 102 receives the HT data and transmits an HT acknowledgement (Ack) to the transmitter-side HT STA 101 in response to the received HT data. An additional HT STA 103 is able to interpret the HT data and the HT Ack. Assuming a duration in which the HT data and the HT Ack are transmitted and received, is set to a Network Allocation Vector (NAV), the medium is considered as being busy. Then, the additional HT STA 103 waits for an DIFS after the lapse of an NAV period of time, and then performs a random backoff, and finally transmits data.
Meanwhile, a legacy station 201 is a station complying with the IEEE 802.11a, 802.11b, or 802.11g protocols but is incapable of interpreting HT data. Thus, after a duration of the HT Ack is checked by physical carrier sensing, the legacy station 201 waits for the duration of an EIFS and then perform a backoff. Thus, the legacy station 201 waits longer than other stations, that is, the transmitter-side HT STA 101, the receiver-side HT STA 102 and the additional HT STA 103, before being assigned media, thereby adversely affecting data transmission efficiency.
The IEEE 802.11 standard specifies a control response frame, such as an ACK, a Request-to-Send (RTS) or a Clear-to-Send (CTS) frame, is transmitted at the same data rate as the directly previous frame. However, if the control response frame cannot be transmitted at the same data rate as the directly previous frame, it must be transmitted at a highest rate in a basic service set (BSS) as specified in the IEEE 802.11 standard. In addition, unlike the legacy format data, the HT data has HT preamble and HT signal fields added thereto, which leads to an increase in the overhead of an PPDU, which may cause the ACK frame to result in deteriorated performance compared to the legacy format PPDU. That is to say, the length of the legacy format PPDU complying with the IEEE 802.11a standard is approximately 20 μs while the length of a newly defined HT PPDU is 40 μs or greater.
Consequently, there exists a need for enhancing performance of network utilization by transmitting legacy format data, e.g., an ACK frame, without an HT preamble when a legacy station cannot interpret data transmitted from an HT station, which may prevent virtual carrier sensing from being performed properly.